1. Field of the Invention
This invention relates generally to protective devices which contain fuses and load switches used in conjunction with power distribution equipment, such as those mounted in metal-clad enclosures, and more particularly, this invention relates to protective devices with removable fuse holders for use in oil-filled power distribution transformers.
2. Description of the Prior Art
To prevent undesirable heating and potentially damaging and dangerous overload conditions, protective devices, which will interrupt the current flow in a circuit, may be inserted in series with a load. Both fuses and circuit breakers are such protective devices. Interrupting characteristics (time to interrupt current flow at various current and temperature levels) differ among protective devices, as does their ability to interrupt current flow. Because of these differences, multiple protective devices may be used together to give the desired interrupting characteristics.
Once current flow is established in a circuit, an inductive effect tends to promote the continuation of current flow. This inductive effect often causes arcing when the circuit is interrupted by separating conductive components. Arcing with the associated high temperatures is generally undesirable for a number of reasons, including potentially damaging combustion and conductive element erosion. The heating associated with high-temperature arcs can be reduced by immersing portions of the circuit where arcing is likely to occur in a liquid insulating medium. Commonly, equipment filled with such an insulating liquid is referred to as "oil-filled equipment".
Oil filling of transformers, which also reduces their operating temperature, is frequent in power distribution systems. Power distribution systems tend to operate at relatively high voltages in order to reduce conductor expenses. Because of such high voltages, current is more likely to flow along undesired paths. Therefore, since various components in a power distribution system must occasionally be moved when high voltages are present, operator safety mandates that the movements be effected with insulated tools, commonly called "hot sticks".
When fuses blow, relatively large reactive forces are often generated. These forces require that fuses be mechanically secured to their holders and that the holders be secured to a support to prevent violent movement. In the event they are not so secured, the fuse, or both the fuse and the fuse holder, may be forcibly propelled, presenting a hazard to equipment and operators. Fuses and their associated receptacles are not generally designed to function as a switch, when the fuse is mechanically withdrawn from the circuit. Should such a withdrawal occur when the circuit is conducting, the fuse may blow. Even when fuse blowing does not occur on withdrawal, damage can be caused to equipment from arcing. Should a conducting circuit be interrupted by withdrawal of a fuse by an operator, the operator is exposed to some risk because of his proximity to components at high-voltage levels. When a fuse is inserted into a high-voltage circuit and completes the conductive path, similar adverse effects exist. It is therefore desirable to provide a means independent of a fuse to complete and interrupt the circuit in which the fuse is located, such as by using a circuit breaker or switch in series with the fuse. It is also desirable, in those instances where the fuse is located in a grounded metal enclosure, to have the fuse easily removable from the interior of the grounded metal enclosure without opening the enclosure.
The protection afforded to the operator by using a circuit breaker or switch with the fuse is diminished if it is possible for an operator to withdraw the fuse without first interrupting current flow therethrough. To prevent such a possibility, some arrangements use load-break connectors which must be removed from an enclosure containing the fuse before safe access to the fuse may be achieved. Load-break connectors are socket and pin arrangements, which an operator disengages. In common with normal load switches, a load break connector speed of interruption, and therefore arc duration, is determined by the speed of actuation by the operator. Since most components and equipment are damaged by an arc of any duration, it is desirable to ensure that any arc drawn is of short duration and that any such arc is drawn between components which are resistant to arc erosion. To ensure short arc duration which is independent of the speed of actuation by the operator, quick-break switches are often used. Such quick-break switches have components which are resistant to arc erosion.
There are prior art devices having a removable fuse holder which cannot function to mechanically interrupt a circuit while it is being withdrawn from its enclosure. These devices require the removal of load-break connectors, or the rotation of fuse carrier contacts, before withdrawal can commence. Their lack of load-break switches, particularly of the quick-break type, is a disadvantage. Other devices include switches in conjunction with fuse holders, removable from the outside of an enclosure or otherwise, but fail to ensure that the switch opens the circuit before removal can be attempted. Such a device for use with a power distribution transformer of the oil-filled type, and which incorporates a quick-break switch, is disclosed in U.S. Pat. No. 3,292,048--Swoish et al., issued on Dec. 13, 1966 for a "Protected Electrical Transformer" and assigned to the same assignee as the present invention.